The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the calibration of the x-ray detector channels.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane." The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional "3rd generation" CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display. The accuracy of this reconstruction is dependent on consistent attenuation measurements of the x-rays by the detector elements throughout the scan. Changes in detector gain produce ring artifacts in the reconstructed image.
The gain stability of various x-ray detector materials and associated electronics can vary considerably. The gains of the individual detector channels in a 3 rd generation CT system are periodically calibrated by performing an "air scan" in which the x-rays unattenuated by an object are measured. System integrity depends on a relatively stable gain between these periodic calibrations. 0n the other hand, because the detectors receive unattenuated x-rays during each scan of an object in a 4th generation CT system, the detector channel gains can be recalibrated during each scan. As a result, some attractive x-ray detector materials like CdTe or CdW0.sub.4 may perform in 4 th generation CT systems but have limited use in 3 rd generation systems because of their gain instability.